During development of the central nervous system (“CNS”), multipotent neural precursor cells, also known as neural stem cells, proliferate and give rise to transiently dividing progenitor cells that eventually differentiate into the cell types that compose the adult brain. Stem cells (from other tissues) have classically been defined as having the ability to self-renew (i.e., form more stem cells), to proliferate, and to differentiate into different phenotypic lineages. In the case of neural stem cells, this includes neurons, astrocytes and oligodendrocytes. Neural stem cells have been isolated from several mammalian species, including mice, rats, pigs and humans. See, e.g., WO 93/01275, WO 94/09119, WO 94/10292, WO 94/16718 and Cattaneo et al., Mol. Brain Res., 42, pp. 161-66 (1996), the disclosures of which are herein incorporated by reference in their entireties. The main function of oligodendrocytes is the myelination of axons in the central nervous system of higher vertebrates. Oligodendrocyte precursor cells (OPCs) precede oligodendrocytes.